For many applications including both military and commercial, there is a requirement to be able to identify the type, size, and other characteristics of a vehicle as it travels over terrain. Especially in battlefield situations, it is important to be able to deploy a number of sensors which are unattended and to be able to track and classify the movement of vehicles through the area guarded by the sensors.
In the past, it has been the practice to obtain an acoustic signature of known vehicles and to compare the received acoustic signals with known signatures in order to ascertain, for instance, the type of vehicle by the sounds that it emits.
For instance, if one is to listen to the sounds from a tank of a predetermined make, one could look at the lines in the noise spectrum of the tank and identify it by the pattern of these lines.
The problem with such identification methods is that one has to have seen the vehicle beforehand and to have been able to measure its acoustic output. For most motorized vehicles utilizing internal combustion engines, by far the loudest sounds which emitted from the vehicle are those associated with the motor, and more particularly with the sounds which are emitted at the end of the muffler. However, the sounds that are made by a particular vehicle vary by the muffler that is utilized, and even, for instance, by the sounds made by a defective muffler.
Attempts have been made also to categorize the type of vehicle by the total amount of sound that it generates. However, whether one is obtaining an acoustic signature or one is obtaining the absolute magnitude of the sound, there are a number of things which affect the measurement. First is directivity in which the sound and sound patterns vary depending on the view of the object from different directions. While it is true that if the sound is emanating from a muffler, it is noted that the wave of the sound is much greater than the length of the muffler. This tends to provide a unidirectional pattern. However, there are some instances when the sound pattern is not unidirectional. It is noted that some vehicles have sound asymmetries which are in fact directional in nature. This therefore results in a directional signature and one which would require mapping to signatures derived from the vehicle at a number of locations around the vehicle.
The second problem is that the absolute value of the sound is more or less loud depending on how far one is from the source. One therefore needs to be able to derive an absolute level referenced to distance.
In addition to acoustic energy emitted by the vehicle, there is also seismic energy which is produced. Especially for heavy tracked vehicles such as tanks, the seismic energy can help in classifying what type of vehicle is making the seismic wave. For instance, the impact of the wheels or the tank treads on the ground as it is propelling the vehicle result in different seismic levels and signatures.
It is, however, to be noted that the seismic wave varies with the particular terrain both in terms of the consistency of the terrain, be it sand, dirt, rock, etc. but also in the near field and the far field. In general, for seismic energy in the 10-50 Hz range, the near field is defined as between 0 and 200 meters, whereas the far field is defined as between 200 meters and up. Seismic energy in the near field is characterized by a body wave, meaning the energy is projected into the earth and reflected back by the various layers or strata at the various locations.
The body wave, however, decreases in energy significantly after about 200 meters and therefore is not useful for seismic classification at such long ranges. However, a surface wave developed by the vehicle running over the terrain is useful in the far field to identify the seismic source level.
Because the variability of the strata and the types of terrain over which the vehicle is traversing, identification by seismic energy alone is quite difficult. Since both acoustic energy and seismic energy decay as 1/distance or 1/distance2 respectively, in order to use absolute values of either of the acoustic or seismic levels arriving at a sensor, it is necessary to know the distance of the vehicle from the sensor so as to appropriately apply the appropriate exponential decay factor to collected data, both for seismic and acoustic sources.
While it is relatively simple to apply attenuation factors to acoustic data, attenuation of seismic waves in the ground is dependent upon the terrain itself. Thus the absolute level of an acoustic signal can be calculated from a reference level and a factor of 10−α R/R which involves the distance of the source from the sensing device.